Fiber-Optic and Satellite Communications

By Gilbert Held

Chapter 7 from Understanding Data Communications, 6th Edition, published by New Riders Publishing

This chapter is about two relatively recently-developed communications technologies that provide the potential to revolutionize the manner by which we communicate: fiber-optic and satellite communications. Although both technologies date to the 1960s, only within the past decade have they been extensively used for communications. This chapter details some of the history of the development of both forms of communications and explains why these techniques generate so much excitement in the communications world. First you will examine why fiber-optic systems work and will learn some of the terms used to describe their operation. Some current terrestrial and undersea fiber-optic systems will be described. Later you will look at the application of fiber optics to local area data networks. Finally, you will study some of the techniques and considerations in transmitting data via geostationary and low-earth orbit satellites.

Introduction and Historical Perspective

Alexander Graham Bell was a very curious and inventive man. In 1880, four years after he invented the telephone, he patented an "apparatus for signaling and communicating, called Photophone." This device, illustrated in Figure 7.1, transmitted a voice signal over a distance of 2000 meters using a beam of sunlight as the carrier. As he spoke into the Photophone, the speaking trumpet vibrated the mirror, varying the light energy that reflected onto the photovoltaic cell. The electric current produced by the cell varied in conjunction with the varying light energy.

The photophone demonstrated the basic principle of optical communications as it is practiced today. The two requirements for commercial success, however, were almost a hundred years away. These requirements were a powerful and reliable light source and a reliable and low-cost medium for transmission.

In 1960, the laser was recognized as the long-sought light source, and systems were tried using both the atmosphere and beam waveguides as the transmission medium. The application of a glass fiber with a cladding was proposed in 1966, and by 1970, fibers with losses of only 20 decibels per kilometer (dB/km) were demonstrated. Since then, progress in the invention and application of fiber optics has been startling. Fibers with losses of less than 0.2 dB/km have been demonstrated in the laboratory (in 1979), as have systems that can transmit at data rates in excess of 400 million bits per second (Mbps) over distances in excess of 100 km without repeaters or amplifiers. Advances in fiber optics began to threaten to make satellite systems obsolete for some kinds of communications (point-to-point where large bandwidths are required, such as transoceanic telephone systems) only 15 years after the satellite systems were commercially employed as the communications systems of the future. Today modern optical fibers are capable of transporting information at data rates exceeding several Gbps. In fact, applying the principle of frequency division multiplexing to light in the form of wavelength division multiplexing (WDM) enables a single fiber to transport up to ten multi Gbps transmissions! To put this in context with something we might appreciate, this is equivalent to transporting the contents of the Library of Congress in perhaps a second!

Optical-fiber transmission has come of age as a major innovation in telecommunications. Such systems offer extremely high bandwidth, freedom from external interference, immunity from interception by external means, and cheap raw materials (silicon, the most abundant material on earth).

Fundamentals of Fiber-Optic Systems

Optical fibers guide light rays within the fiber material. They can do this because light rays bend or change direction when they pass from one medium to another. They bend because the speed of propagation of light in each medium is different. This phenomenon is called refraction. One common example of refraction occurs when you stand at the edge of a pool and look at an object at the bottom of the pool. Unless you are directly over the object, as shown in Figure 7.2a, it appears to be farther away than it really is, as indicated in Figure 7.2b. This effect occurs because the speed of the light rays from the object increases as the light rays pass from the water to the air. This causes them to bend, changing the angle at which you perceive the object. We can obtain an appreciation for the manner by which light flows by focusing our attention upon Snell's Law.

Figure 7.2: Bending of light rays.

Snell's Law

How optical fibers work can be explained by Snell's Law, which states that the ratio of the sine of the angle of incidence to the sine of the angle of refraction is equal to the ratio of the propagation velocities of the wave in the two respective media. This is equal to a constant that is the ratio of the refractive index of the second medium to that of the first. Written as an equation, Snell's Law looks like this:

sin A1 V1 n2

= = K =

sin A2 V2 n1

In this equation:

A1 and A2 are the angles of incidence and refraction, respectively

V1 and V2 are the velocities of propagation of the wave in the two media

n1 and n2 are the indices of refraction of the two media

The parameters are demonstrated graphically in Figure 7.3. In each case, A1 is the angle of incidence, and A2 is the angle of refraction. The index of refraction of material 1, n1, is greater than the index of refraction of material 2, n2. This means that the velocity of propagation of light is greater in material 2 than in material 1.

Figure 7.3a demonstrates how a light ray passing from material 1 to material 2 is refracted in material 2 when A1 is less than the critical angle. Figure 7.3b demonstrates the condition that exists when A1 is at the critical angle and the angle A2 is at 90 degrees. The light ray is directed along the boundary between the two materials.

As shown in Figure 7.3c, any light rays that are incident at angles greater than A1 of Figure 7.3b will be reflected back into material 1 with angle A2 equal to angle A1. The condition in Figure 7.3c is the one of particular interest for optical fibers, and it is discussed further in later sections of this chapter.

Fiber Composition

An optical fiber is a dielectric (nonconductor of electricity) waveguide made of glass or plastic. As shown in Figure 7.4, it consists of three distinct regions: a core, the cladding, and a sheath or jacket. The sheath or jacket protects the fiber but does not govern the transmission capability of the fiber.

Figure 7.3: The index of refraction.

The index of refraction of the assembly varies across the radius of the cable, and the core has a constant or smoothly varying index of refraction called nc. The cladding region has another constant index of refraction called n. The core possesses a high refractive index, whereas the cladding is constructed to have a lower refractive index. The result of the difference in the refractive indices keeps light flowing through the core after it gets into the core, even if the fiber is bent or tied into a knot. For a fiber designed to carry light in several modes of propagation at the same time (called a multimode fiber), the diameter of the core is several times the wavelength of the light to be carried. Wavelength is a measure of the distance between two cycles of the same wave measured in nanometers (nm) or billionths of a meter, and the cladding thickness will be greater than the radius of the core. Following are some typical values for a multimode fiber:

An operating light wavelength of 0.8 micrometers mm.

A core index of refraction nc of 1.5.

A cladding index of refraction n of 1.485 (=0.99 [infin] nc).

A core diameter of 50, 62.5, or 100 mm.

A cladding thickness of 37.5 mm.

The clad fiber would have a diameter of 125 mm, and light would propagate as shown in Figure 7.5.

A light source emits light at many angles relative to the center of the fiber. In Figure 7.5, light ray A enters the fiber perpendicular to the face of the core and parallel to the axis. Its angle of incidence A1 is zero; therefore, it is not refracted, and it travels parallel to the axis. Light ray B enters the fiber core from air (n1 = 1) at an angle of incidence of A1B and is refracted at an angle A2B because n2 is greater than n1. When light ray B strikes the boundary between the core and the cladding, its angle of incidence, A1'B, is greater than the critical angle. Therefore, the angle of refraction, A2'B, is equal to A1'B, and the light ray is refracted back into the core. The ray propagates in this zigzag fashion down the core until it reaches the other end.

If the angle of incidence, A1C, is too large, as it is for light ray C, the light ray strikes the boundary between the core and the cladding with an angle of incidence, A1'C, less than the critical angle. The ray enters into the cladding and propagates into, or is absorbed into, the cladding and jacket (which is opaque to light).

Figure 7.4: Optical-fiber construction.

Figure 7.5: Light ray paths in multimode fiber.

Modal Delay

For optical fibers in which the diameter of the core is many times the wavelength of the light transmitted, the light beam travels along the fiber by bouncing back and forth at the interface between the core and the cladding. Rays entering the fiber at differing angles are refracted varying numbers of times as they move from one end to the other and consequently do not arrive at the distant end with the same phase relationship as when they started. The differing angles of entry are called modes of propagation (or just modes), and a fiber carrying several modes is called a multimode fiber. Multimode propagation causes the rays leaving the fiber to interfere both constructively and destructively as they leave the end of the fiber. This effect is called modal delay spreading.

Because most optical communications systems transmit information in digital form consisting of pulses of light, the effect of modal delay spreading limits the capability of the fiber to transport recognizable pulses. This is because modal delay spreading broadens the pulses in the time domain, as illustrated in Figure 7.6. The effect of pulse spreading is to make it difficult or impossible for an optical receiver to differentiate one pulse from another after a given transmission distance. Thus, after a predefined transmission distance, a multimode fiber either causes a very high error rate or precludes the capability of the pulse to be recognized and terminates the capability of the cable to be used for communications.

Figure 7.6: Pulse spreading.

If the diameter of the fiber core is only a few times the wavelength of the transmitted light (say a factor of 3), only one ray or mode will be propagated, and no destructive interference between rays will occur. These fibers, called single-mode fibers, are the media that are used in most transmission systems. Figures 7.7a and 7.7b show the distribution of the index of refraction across, and typical diameters of, multimode and single-mode fibers. One of the principal differences between single-mode and multimode fibers is that most of the power in the multimode fiber travels in the core, whereas in single-mode fibers, a large fraction of the power is propagated in the cladding near the core. At the point where the light wavelength becomes long enough to cause single-mode propagation, about 20 percent of the power is carried in the cladding, but if the light wavelength is doubled, more than 50 percent of the power travels in the cladding.

Refractive Index

Fiber can also be classified by its type of refractive index. Figure 7.7 illustrates a few of the classifications, which are outlined here.

Stepped index fiber. The fiber core has a uniform refractive index throughout with a sudden change of the refractive index at the core-cladding boundary.

Graded index fiber. The fiber core which has a refractive index that gradually decreases as the distance from the center of the fiber increases.

Single-mode fiber. Also known as monomode, this has a uniform refractive index. This type of fiber only permits a single light ray to pass through the cable.

Graded index multimode fiber. The index of refraction varies smoothly across the diameter of the core but remains constant in the cladding. This treatment reduces the inter-modal dispersion by the fiber because rays traveling along a graded-index fiber have nearly equal delays.

Figure 7.7: Refractive index profiles.

Other refractive-index profiles have been devised to solve various problems, such as reduction of chromatic dispersion. Some of these profiles are shown in Figure 7.8; the step and graded profiles are repeated for comparison.

Figure 7.9 compares the flow of light through step-index, graded-index, and single-mode fiber. A step index fiber typically has a core diameter between 100 mm and 500 mm. A graded-index fiber commonly has a core diameter of 50 mm or 62.5 mm, while single-mode fibers have core diameters between 8 and 10 mm. Both stepped-index and graded-index fiber support multimode transmission.

Figure 7.8: Different refractive index profiles for optical fibers.

Figure 7.9: Light flow through different refractive index fibers.

Bandwidth

The limitations on bandwidth in fiber-optic systems arise from two main sources: modal delay spreading and material dispersion. Modal delay spreading, which was previously described, is evident primarily in multimode fibers. Material dispersion arises from the variation in the velocity of light through the fiber with the wavelength of the light.

If the light source, such as a light-emitting diode (LED), emits pulses of light at more than one wavelength, the different wavelengths travel at different velocities through the fiber. This causes spreading of the pulses. At a typical LED wavelength of 0.8 mm, the delay variation is about 100 picoseconds (ps) per nanometer (nm) per km. If the width of the spectrum emitted by the LED is 50 nm, pulses from the source are spread by 5 nanoseconds (ns) per km. This limits the modulation bandwidth product to about 50 to 100 MHz/km. Fortunately, at certain wavelengths (near 1.3 and 1.5 mm for some types of fibers), there is a null in the material dispersion curve, giving much better modulation bandwidth performance. Figure 7.10 shows the relationship of loss in doped silicon glass fibers versus light wavelength. Most current development work is aimed at making fibers, light sources, and detectors that work well at the loss nulls at 1.3 and 1.5 mm.

Figure 7.10: The net spectral loss curve for a glass core.

Reading Cable Measurements

When examining fiber cable, you will commonly note a dual numeric specified for the fiber, such as 62.5/125. The first number references the core size, and the second number references the cladding diameter in microns. Three common multimode fiber cables in use are 62.5/125, 50/125, and 100/140.

Attenuation

The loss in signal power as the light travels down the fiber is called attenuation. Attenuation in the fibers is controlled mainly by four factors: radiation of the propagated light, called scattering; conversion of the light energy to heat, called absorption; connection losses at splices and joints in the fiber; and losses at bends in the fiber.

Scattering Losses

Scattering occurs due to microscopic imperfections in the fiber, such as the inclusion of water in the glass. The effect of impurities in the transmission medium is evident when we look up at the sky and see a blue color. In fact, deep space has no color (appears as black), but due to the scattering of sunlight by the dust in the atmosphere, the sky appears bright blue.

There is a limit below which scattering cannot be reduced, no matter how perfectly the glass fiber is made, because of irregularities in the molecular structure of glass. This limit, called the Rayleigh scattering limit, has a strong wavelength dependence (1/l4). Thus, as the wavelength of the light source increases, the effect of Rayleigh scattering on optical loss is reduced. This effect is shown graphically in Figure 7.10. For light with a wavelength of 0.8 mm it is about 2.9dB/km. At a wavelength of 1.3 mm, the value is about 0.3dB/km, and at 1.55 mm wavelength, the limit is about 0.15dB/km. Commercially available glass fibers exhibit losses of about 3.5dB/km at 0.8 mm, and 0.7 to 1.5dB/km at 1.3 and 1.5 mm. There is less attenuation through 6 meters (about 20 feet) of good quality optical-fiber glass than through an ordinary clean windowpane.

Absorption Losses

Absorption refers to the conversion of the power in the light beam to heat in some material or imperfection that is partially or completely opaque. This property is useful, as in the jacket of the fiber, to keep the light from escaping the cable, but it is a problem when it occurs as inclusions or imperfections in the fiber. Current fiber-optic systems are designed to minimize intrinsic absorption by transmitting at 0.8, 1.3, and 1.5 mm, where there are reductions in the absorption curve for light.

Connection Losses

Connection losses are inevitable. They represent a large source of loss in commercial fiber-optic systems. In addition to the installation connections, repair connections will be required because experience has shown that a typical line will be broken accidentally two or three times per kilometer over a 30-year period. The alignment of optical fibers required at each connection is a considerable mechanical feat. The full effect of the connection is not achieved unless the parts are aligned correctly. The ends of the fibers must be parallel within one degree or less, and the core must be concentric with the cladding to within 0.5 mm. Production techniques have been developed to splice single-mode fibers whose total diameter is less than 10 mm by using a mounting fixture and small electric heater.

The heater is usually an electric arc that softens two butted fiber ends and allows the fibers to be fused together. Due to the cost of an electric arc and the time required to let the heated ends cool, other methods to connect broken fibers, such as mechanical splices and couplers, are more commonly used. Mechanical splicing is based on the use of a mechanical clamp to permanently splice together two fibers. This task is accomplished with a portable workstation that is used to prepare each fiber end. That preparation includes stripping a thin layer of plastic coating from the fiber core before its splicing. The typical mechanical splice loss is approximately .15dB. In comparison, the use of a connector enables fibers to be glued to the device by a special epoxy after each fiber is stripped to its proper dimension. The connector provides a splice that joins the two fibers to enable light to pass from one fiber to the other. Although connector loss can range from .25 to 1.5dB, use of connectors is preferred for many labor-intensive applications because the process is relatively quick and can usually be accomplished in 30 minutes or less with a minimal amount of equipment.

Bending Losses

Bending an optical fiber is akin to playing crack-the-whip with the light rays. As light travels around the bend, the light on the outside of the bend must travel faster to maintain a constant phase across the wave. As the radius of the bend is decreased, a point is reached where part of the wave would have to travel faster than the local speed of light—an obvious impossibility. At that point, the light is lost from the waveguide. For commercial single-mode fiber-optic cables operating at 1.3 and 1.5 mm, the bending occurring in fabrication (the cables are made with the fibers wound spirally around a center) and installation does not cause a noticeable increase in attenuation.

Numerical Aperture and Acceptance Angle

The numerical aperture of the optical fiber is a measure of its light-gathering capability (much like the maximum f-stop of a camera lens). The numerical aperture is defined as the maximum angle of incidence of a ray that is totally reflected at the core/cladding interface. Mathematically, the numerical aperture, NA, is expressed in this way:

NA = n2 core - n2 cladding

The optical power accepted by the fiber varies as the square of the numerical aperture, but unlike the camera lens f-stop, the numerical aperture does not depend on any physical dimension of the optical fiber.

The acceptance angle is the maximum angle that an entering light ray can have relative to the axis of the fiber and still propagate down the fiber. A large acceptance angle makes the end alignment less critical when fibers are being spliced and connected.

Fiber-Optic Subsystems and Components

Several components provide the foundation for the construction of fiber-optic systems and subsystems. Those components include fiber production, light sources, and light detectors.

Fiber Production

Optical fibers are fabricated in several ways, depending on the vendor and the purpose of the system. The core and cladding regions of the fiber are doped to alter their refractive indices. This doping is carried out by heating vapors of various substances such as germanium, phosphorus, and fluorine, and depositing the particles of resulting oxidized vapor or "soot" on high-quality fused-silica glass mandrels, called preforms. The preforms are large-scale version of the core and cladding that are then heated to a taffy-like consistency and drawn down into the actual fiber. The core and cladding dimensions have essentially the same relationship in the final fiber as in the preform. Deposition of the dopants is done in one of three standard ways: outside, inside, and axial vapor deposition.

Light Sources

Light sources for fiber-optic systems must convert electrical energy from the computer or terminal circuits feeding them to optical energy (photons) in a way that allows the light to be coupled effectively to the fiber. Two such sources currently in production are the surface light-emitting diode (LED) and the injection laser diode (ILD).

Light-Emitting Diodes

A cross section of a surface LED is shown in Figure 7.11. It emits light over a relatively broad spectrum, and it disperses the emitted light over a rather large angle. This causes the LED to couple much less power into a fiber with a given acceptance angle than does the ILD. Currently, LEDs are able to couple about 100 microwatts (mW) of power into a fiber with a numerical aperture of 0.2 or more and a coupling efficiency of about 2 percent. The principal advantages of LEDs are low cost and high reliability.

Injection Laser Diodes

A cross section of a typical ILD is shown in Figure 7.12. Because of its narrow spectrum of emission and its capability to couple output efficiently into the fiber lightguide, the ILD supplies power levels of 5 to 7 milliwatts (mW). At present, ILDs are considerably more expensive than LEDs, and their service life is generally less by a factor of about 10. Other disadvantages of laser diodes are that they must be supplied with automatic level control circuits, the laser power output must be controlled, and the device must be protected from power supply transients.

Light Detectors

At the receiving end of the optical communications system, the receiver must have very high sensitivity and low noise production. To meet these requirements, there is a choice of two types of devices to detect the light beam, amplify it, and convert it back into an electrical signal: the integrated p-i-n field-effect transistor (FET) assembly and the avalanche photodiode (APD). In the p-i-n FET device, a photodiode with unity gain (the p-i-n device) is coupled with a high-impedance front-end amplifier. This device combines operation at low voltage with low sensitivity to operating temperature, high reliability, and ease of manufacture.

The avalanche photodiode produces a gain of 100 or more; however, it also produces noise that might limit the receiver sensitivity. The APD devices require high voltage bias that varies with temperature. Receivers using APDs are so sensitive that they require as few as 200 phototons to be detected at the receiver per bit transmitted at data rates of 200 to 400Mbps.

Wavelength Multiplexing

A combination of the single-mode fiber (low dispersion by the transmission medium), narrow output spectrum (power concentration at a single frequency), and narrow dispersion angle (good power coupling) from ILDs makes possible the extreme bandwidth-distance characteristics given for systems at the beginning of this chapter. The narrow ILD emission spectrum also makes it possible to send several signals from different sources down the same fiber by a technique called wavelength multiplexing. The capability to multiplex several analog signals in the frequency domain has been described in detail in Chapter 6, "Multiplexing Techniques." As illustrated in Figure 7.13, wavelength multiplexing at optical frequencies is the equivalent of FDM at lower frequencies. Light at two or more discrete wavelengths is coupled into the fiber and each wavelength carries a channel at whatever modulation rate is used by the transmission equipment driving the light source. Thus, the information capacity of each fiber is doubled or tripled.

Transmission Systems

During the past decade, most carrier transmission systems have moved from the use of copper wire to optical fiber. In fact, by mid-1994, almost 100 percent of long-distance communications was carried over optical transmission systems.

An AT&T study in 1978 noted a striking advantage of installing a digital transmission system over comparable analog systems: a $900 savings per circuit termination when interconnecting multiple digital switching machines. The savings came from the difference in requirement for terminal multiplexing equipment. The AT&T FT3C lightwave system was devised to provide the most economical digital transmission system possible with the current state of the fiber-optic art. It used wavelength multiplexing techniques to send three 90Mbps signals over the same fiber, giving more than 240,000 digital channels at 64,000bps in a cable containing 144 optical fibers.

The first application of the system was in the Northeast Corridor project by AT&T, between Boston and Washington, and in the North/South Lightwave Project on the West Coast of the United States by Pacific Telesis, between San Francisco and San Diego. Figure 7.14a is a map of the Northeast Corridor system, which contains 78,000 fiber-kilometers of lightwave circuits. Figure 7.14b is a map of the North/South Lightwave Project. The two systems were placed in service in 1983 and have been expanded considerably since then.

The significant advantages associated with the use of fiber-optic transmission systems resulted in tens of thousands of miles of fiber being installed during the 1980s. By early 1991, U.S. Sprint had converted 100 percent of its inter-city transmission facilities from microwave to fiber. A few years later, AT&T and MCI Communications completed the conversion of their systems to fiber.

During the mid to late 1990s, the growth in the use of the Internet and other data applications resulted in the use of traditional long-distance communications carriers such as AT&T, MCI, and Sprint being supplemented by a number of newly formed communications carriers such as IXC Communications, Quest Communications, and Level 3 Communications. The newly formed companies installed more than 50,000 route miles of fiber along gas, railroad, and electric utility right of ways to develop their own long distance networks. In the face of competition, the traditional long distance communications carriers spent billions of dollars upgrading their previously installed fiber-based infrastructure to compete with the newly formed carriers. Upgrading of existing fiber included new optical transmitter/receivers capable of supporting data rates as high as 2.4Gbps, as well as the use of wavelength division multiplexing which permits up to four separate optical channels, each transporting a 2.4Gbps data stream to provide a 9.6Gbps composite transmission capability over existing fiber links. These same fiber links may have been hard pressed to support a data rate of 300Mbps earlier in the decade.

Digital Transmission Systems that use state-of-the-art Fiber Optics

Figure 7.14: Fiber-optic networks.

SONET

Advances in the use of fiber-optic transmission systems resulted in a requirement for standards to enable interoperability between interexchange carriers and telephone companies. In addition, a considerable growth in communications from companies and government agencies resulted in a requirement to define the interface of commercial communications equipment to an evolving optical network. The resulting standard, known as the Synchronous Optical Network (SONET) in North America and Synchronous Digital Hierarchy (SDH) in Europe, represents a transport vehicle capable of supporting data rates in the gigabit range, optical interfaces, network management, and diagnostic testing methods.

Until SONET standards were developed, there was a void in compatibility between fiber terminal equipment operating at rates above the DS3 transmission rate of 44.736Mbps. That operating rate is formed by a communications carrier using a device known as an M13 multiplexer to combine 28 DS1 channels into a DS3 signal. The resulting DS3 signal is asynchronous because each DS1 signal is independently timed. Although each DS1 signal includes 8000 bits per second for framing, the resulting multiplexed DS3 signal includes three intermixed framing signals, which makes it almost impossible to locate an individual DS0 signal within the DS3 signal. Thus, to remove one PCM digitized voice signal in a carrier's DS3 transmission hierarchy, the carrier typically had to first demultiplex the DS3 signal, requiring additional equipment and adding to the cost of the carrier's infrastructure. Recognizing this problem, the developers of the SONET standard developed a frame structure that enables lower-speed channels within a higher-speed signal to be easily removed from or added to the signal. This process is known as drop (removal of the signal on a channel) and insert (addition of a signal on a channel).

Frame Structure

As previously indicated the basic SONET frame consists of nine rows of bytes with 90 bytes per row, and 8,000 of these frames are transmitted each second. This arrangement results in a composite data rate of 51.84Mbps. Because 27 bytes of each 810-byte frame represent overhead, the payload of the basic SONET frame is limited to 50.112Mbps, of which 2.3Mbps represents overhead. That overhead includes positioning information that enables a single DS0 channel to be identified and data easily extracted or inserted into the channel position. These characteristics of SONET's frame structure are illustrated in Figure 7.15.

This also means that a DS3 signal representing 28 DS1 signals can be carried within the basic SONET frame, and each of the 672 DS0 channels within the 28 DS1 signals can have data easily removed (dropped) or added (inserted).

The basic SONET frame illustrated in Figure 7.15 that occurs 8000 times per second is referred to as a Synchronous Transport Signal-Level 1, or STS-1. SONET is called synchronous because the frame is synchronized with respect to each input DS0 channel through the use of pointers in the 27 bytes of overhead per frame, which results in a synchronous multiplexing format. In addition to pointers, a significant portion of the overhead bytes in the STS-1 frame are reserved for management data, enabling network management functions to include loopback, bit error rate testing, and collection and reporting of performance statistics to be carried within the SONET frame.

Figure 7.15: The SONET frame structure.

The Optical Interface

One of the major elements included in the SONET standard is its set of defined optical interfaces. Until the development of SONET, each manufacturer designed its fiber terminal device to its own optical signal interface. This "do-it-yourself" approach prevented the interconnection of terminal devices from different vendors with a common fiber backbone. Under SONET, 256 OC (optical carrier) optical interfaces are defined, although the current standard explicitly calls for the use of 8 interfaces. Those interfaces, listed in Table 7.1, define the SONET digital signal hierarchy. Note that the OC-1 level represents an STS-1 signal, whereas higher levels represent multiplexed STS-1 signals.

Although SONET primarily represents a standard for optical interoperability among public network transmission providers, it also furnishes the capability for large organizations and government agencies to provide an STS-1 signal to the central office of a public network provider. This is accomplished using an optical multiplexer connected to a fiber routed from the customer premise to the carrier's central office. Figure 7.16 illustrates the use of an STS-1 signal to the public network. Note that because of the pointers imbedded into the SONET frame, the carrier can easily break out previously multiplexed signals originating at the customer's optical multiplexer without having to demultiplex and remultiplex the data stream, which would be required with a DS3 signal. Eventually, SONET should reduce the cost of transmission for large network users and increase network reliability as fiber is routed directly to the customer premises.

Table 7.1 The SONET Signal Hierarchy

Level

Line Rate

DS3 Channels

OC-1

51.84Mbps

1

OC-3

155.52Mbps

3

OC-9

466.56Mbps

9

OC-12

622.08Mbps

12

OC-18

933.12Mbps

18

OC-24

1.244Gbps

24

OC-36

1.866Gbps

36

OC-48

2.488Gbps

48

Figure 7.16: Using SONET to connect public and private networks.

International Systems—The SL Underwater Cable

The advent of optical-fiber technology for undersea cables provides point-to-point channel capacity equal to satellite systems at reduced cost and without long transmission delays, unstable environmental interference, induced noise, and broadcast of potentially sensitive information to half the world.

Undersea cable systems have some understandably difficult environmental requirements. The environment includes pressures of 10,000 pounds per square inch (psi) at depths of 7,300 meters, salt water, and the possibility of mechanical damage from anchors and earth movement in shallow waters. An important requirement for these systems is that the regenerator spacing be as wide as possible to cut down on the system failure probability and the power requirements, because power must be fed from the ends of the cable.

A schematic of the SL Undersea Lightguide System is shown in Figure 7.17. It is composed of a high-voltage power supply, a supervisory terminal, a multiplexer with inputs for several types of information, the cable light source, the cable itself, and the repeaters. The cable is made of a central core and a surrounding support, as shown in Figure 7.18. The core has an outside diameter of 2.6 mm and consists of 12 optical fibers wound helically around a central copper-clad steel wire called a kingwire, all embedded in an elastomeric substance and covered with a nylon sheath. This assembly is in turn covered with steel strands, a continuously welded copper tube, and an insulation of low-density polyethylene for electrical insulation and abrasion resistance. The outside diameter of the completed cable is 21 mm (about 0.8 in.).

Figure 7.17: The SL Undersea Lightguide System.

Figure 7.18: Embedded fiber core cable.

The fibers are single-mode optical lightguides operating at 1.312 mm. The data rate on each fiber pair is 280Mbps, and repeaters are spaced every 35 km. The total capacity of the system is more than 35,000 two-way voice channels. Inputs from binary data sources are multiplexed directly into the stream. Analog channels are first converted to binary by the adaptive delta modulation technique, and are then processed by a type of digital circuit (Digital TASI) that interleaves inputs from a number of input speech channels onto a smaller number of output channels. The light sources for the systems are ILDs operating near 1.3 mm with an average output power of 1 mW (0 dBm). Light detectors are indium-gallium-arsenide (InGaAs) p-i-n diode receivers followed by silicon bipolar transimpedance amplifiers. Three spare laser diodes, which can be switched in remotely if a failure were to occur, are provided per circuit.

By the late 1980s, several fiber-optic undersea cables were laid across the Atlantic, linking New York to England, France, and Spain via light transmission. The successful operation of those systems resulted in communications carriers expanding their plans for fiber-optic undersea cables. During the early 1990s, several trans-Pacific and trans-Mediterranean fiber-optic cables were installed. By the mid-1990s, most voice, data, and video transmissions between the United States and Europe, Japan, and Australia were routed via fiber-optic cable between continents.

As the use of fiber-optic cable has increased, a corresponding decrease has resulted in the use of geostationary satellites for voice transmission between fixed locations. The primary reason for the wide acceptance of fiber-optic cable over geostationary satellites is the immunity of the fiber-optic cable to electromagnetic interference. An additional factor was the elimination of one-half second or longer delays associated with the use of geostationary satellites caused by the 50,000 plus miles the signal had to travel during transmission. Today, most geostationary satellites are used for television and data transmissions that are insensitive to transmission delays. However, a new series of low earth-orbiting satellites during the late 1990s (described in the next section in this chapter) significantly enhance the use of satellites for voice communications. Those satellites support a new class of mobile communications referred to as Personal Communications Service (PCS), which is also described later in this chapter.

LAN Applications

The use of fiber in local area networks has considerably expanded over the past decade. During the late 1980s,w fiber was commonly used between electrical-optical repeaters as a mechanism to extend the length of LANs. Currently one LAN referred to as FDDI (Fiber Data Distribution Interface) represents an optical fiber-based local area network that supports data transmission at 100Mbps. FDDI can support transmission distances beyond 50 miles, which makes it suitable for creating campus-wide backbones. Another type of LAN referred to as "Gigabit Ethernet" requires the use of optical fiber on all connections beyond 100 meters due to the extreme difficulty in transmitting very narrow pulses over copper media and having them recognizable at a receiver. As organizations require higher data transfer rates the use of fiber can be expected to grow in local area network applications.

Fiber to the Home

One of the most publicized terms in the field of communications is the so-called digital highway. First promoted by Vice President Gore as a term to represent the interconnections of computer research centers via high-speed optical transmission, the term has been applied to all types of high-capacity communications to include fiber cables routed to the home.

By early 1994, several field tests of fiber to the home were being conducted in California and Florida. Each test was similar in that the large bandwidth capacity of an optical fiber enabled the simultaneous transmission of up to hundreds of television signals along with voice conversations and interactive data channels. Where each test differed was in the scope of interactivity offered to the home consumer and the ability of the consumer to request different functions interactively. Some tests enabled the consumer to order films to be displayed at a predefined time, a feature known as video-on-demand. Other tests primarily focused on providing consumers with online banking, grocery ordering, and catalog shopping. A new term used to define the provider of integrated voice, data, and video is multimedia, although the range of services offered by different multimedia vendors currently varies considerably from vendor to vendor.

Currently a competition between cable TV operators and telephone companies for the multimedia consumer dollar is being waged with competitive technology. Most telephone companies are either conducting field trials or gradually making available digital subscriber line (DSL) technology to their customers. In comparison, cable TV operators are rapidly installing hybrid fiber coax cable combinations, upgrading their infrastructure to support bidirectional communications and offering cable modems to their subscribers.

HFC and Cable Modems

In addition to conducting fiber-to-the-home trials, the cable TV industry has been busy upgrading a good portion of its infrastructure using a hybrid fiber coax (HFC) cable combination. Through the use of HFC between 500 and 3000, cable TV subscribers can be supported by one fiber main trunk node being connected to coax feeder and drop cables. In addition, the large bandwidth of the fiber trunk cable enables the CATV operator to support transmission in the return direction from subscribers in a large service area via the fiber trunk. Thus, an HFC infrastructure provides the opportunity for cable TV operators to provide telephone service, Internet access, and other services that require economical bidirectional transmission.

Figure 7.19 illustrates in schematic form a hybrid fiber coax network. Note that each main trunk consists of a fiber cable, and feeder and drop cables routed to the subscriber continue the use of coax. This method of cabling enables the CATV operator to considerably increase the capacity of the system in an economical manner, because the existing coaxial cable previously routed to subscribers remains in place.

Figure 7.19: A CATV hybrid fiber coax network.

In Figure 7.19, notice that to provide two-way services, CATV operators must upgrade their amplifiers to support bidirectional transmission. In addition, note that the cable TV headend represents the broadcast area where satellite video feeds are received and broadcast to subscribers. Although Figure 7.19 illustrates a hybrid fiber coax network bidirectional transmission necessary to support telephone service, note that interactive TV, Internet applications, and similar applications can be supported on conventional coaxial TV systems that were upgraded to support bidirectional transmission. The key difference between the two is that the use of fiber trunks significantly reduces the number of trunks that must be routed into neighborhoods, and it provides immunity to noise and other transmission impairments resulting from electrical disturbances.

Most cable modems have a receiver that's tunable to the 50–750MHz range in 6MHz increments, enabling the modem to use a full TV channel for data reception. Through the use of 64- or 256-point Quadrature Amplitude Modulation (QAM) using a majority of the bandwidth of the TV channel, a data rate up to 36Mbps can be supported. In the return direction, cable modems operate in the 5–42MHz spectrum and support a 10Mbps upstream rate. See Chapter 5 for a detailed discussion of cable modem technology.

Today, the home multimedia effort is in its infancy, probably equivalent to where the personal computer industry was during the 1970s. Although it's difficult to predict the outcome of current field trials, as well as the range of services that will be offered to consumers, one thing is certain: Regardless of the resulting suite of services offered to the consumer under the labels "video-on-demand," "multimedia," or another term, those services will be transported through the use of fiber-optic cable to the home, either directly or via the use of hybrid fiber coax systems.

Satellite Transmission Systems

Satellite transmission systems provide users with the ability to bypass conventional communications carrier offices, as well as to broadcast information to multiple locations for a nominal cost.

Basic Satellite Technology

Satellite communications systems are basically line-of-sight microwave systems with a single repeater. As stated in Chapter 3, "Messages and Transmission Channels," the satellite is said to be in geostationary orbit when the speed of the satellite is matched to the rotation of the earth at the equator. Because of the great distance of the satellite from the earth (about 22,300 miles) and antenna size limitations that limit focusing capability, the cone of coverage for a single satellite transmitter can be as large as the entire continental United States.

For those transmission services that originate at a single point and flow to many points in one direction, such as television and radio signals, the large area of coverage is ideal. The relatively long delay between the instant a signal is sent and when it returns to earth (about 240 milliseconds –) has no undesirable effect when the signal is going only one way. However, for signals such as data communications sessions and telephone conversations, which go in both directions and are intended to be received at only one other point, the large area of coverage and the delay can cause problems.

Data and telephone conversations usually proceed as a series of messages in one direction that are answered or acknowledged by messages in the other direction. When the delay between the message being sent and the reply or acknowledgment is long, the transmission rate of information slows down. In the case of voice messages, long delays between utterance and reply make the speaker think he or she has not been heard or understood. This leads to requests for repeating (the equivalent of negative acknowledgment in the data world) and increased frustration. There is also a serious privacy issue with communications that are intended for only one destination but are broadcast so that an entire continent can receive them. One of the factors causing the rush to all-digital transmission is the ease of encrypting information in digital form so that when the inevitable interception of broadcast signals occurs, the information intercepted is at least somewhat difficult to decipher.

A satellite transmission system consists of one or more earth stations and a geostationary satellite that can be seen by the stations. Figure 7.20 illustrates the Americatel "teleport," consisting of three large-diameter satellite dishes located in Miami, Florida, that represent earth stations. Americatel is a regional telecommunications carrier that provides voice, data, and facsimile services via a gateway and switching center in Miami to common carriers and private organizations throughout Central and South America. In Central and South America, many subscribers to Americatel use Very Small Aperture Terminals (VSATs) to communicate with North America, bypassing the necessity to use terrestrial lines. Through the use of VSAT facilities in remote areas, such as mining locations, users can receive a modern communications capability that was previously restricted to major metropolitan locations. To facilitate satellite communications, as well as to eliminate interference between transmission and reception, standards govern the use of satellite frequencies. Separate frequencies are assigned for sending to the satellite (the uplink) and receiving from the satellite (the downlink). The general frequency assignments for satellite systems are shown in Table 7.2.

Satellites are equipped with multiple repeater units called transponders. Many systems have 10 or 12 transponders, but a series of international satellites, called INTELSAT VI, has 46 transponders. Transponders are assigned different uses, but in the case of those used for voice or voice-equivalent data communications channels (nominal 4KHz bandwidth), the transponder capacity can be as large as 3,000 channels.

The INTELSAT VI satellite provides a total bandwidth, using frequency reuse techniques, of 3460MHz.

Transponders operate at different carrier frequencies. Currently seven frequency bands are used for most space communications applications. Table 7.3 lists the frequency range assignments for seven space frequency bands. Note that the C-Band and Ku-Band are currently used exclusively for broadcasting purposes, such as for providing HBO, Showtime, and other television broadcasts to wide areas of the world.

Multiple Access Systems

Telephone switching systems and data multiplexers are designed based on the fact that not every telephone or terminal that can send information will do so at the same time; or alternatively, the telephone or terminal user does not need or want to pay for the entire capacity of a channel. These conditions are also true for the users of satellite systems. Several methods have been devised to allow sharing of the satellite and earth station resources among several users so that it appears that the transmission channel is dedicated to each user.

Frequency Division Multiple Access (FDMA) is simply another example of the familiar data and voice transmission technique called frequency division multiplexing (FDM). This technique is used to allocate small portions of a large bandwidth (500MHz for satellite transponders) to individual users. For instance, a telecommunications common carrier in a particular country, say Brazil, might want 132 voice-grade channels for sending voice- and analog-coded data to various other countries. The bandwidth required on the current international satellite systems for this many channels is 10MHz. Because one transponder has a bandwidth of 500MHz, it could accommodate 50 users, each requiring 132 channels. The Brazilian user might be allocated the frequency band between 5990 and 6000MHz for the uplink to the satellite, and the corresponding downlink frequencies would be 3765 to 3775MHz. Other users might be assigned similar portions of the bandwidth in the same transponder. For example, a Portuguese user might be allocated the 6220 to 6230MHz uplink band and the 3995 to 4005MHz downlink band. A Canadian user might operate on the 5930 to 5940MHz uplink band and the 3705 to 3715MHz downlink band. Figure 7.21 illustrates how the three users each have one uplink, all into a single transponder, but all users can receive all three downlinks. This arrangement makes possible simultaneous two-way transmissions between any of the three sites using only a part of one satellite transponder.

Table 7.3 Space Communications Bands and their Frequency Range

Band

Frequency Range

L

1.0–2.0GHz

S

2.01–4.0GHz

C

4.01–8.0GHz

X

8.01–12.0GHz

Ku

12.01–18.0GHz

K

18.01–27.0GHz

Ka

27.01–40.0 GHz

Each user has a permanently assigned channel

Figure 7.21: Multiuser satellite system.

Time Division Multiple Access

Time Division Multiple Access (TDMA) is the equivalent of FDMA, but in the time domain rather than the frequency domain. TDMA works just like time division multiplexing (TDM) for land-based data and voice transmissions, with each satellite transponder having a data rate capacity of between 10 and 100Mbps. While a station is sending on the uplink, the entire data rate capacity of the transponder is available, but then it must stop sending for a short time to allow another station access to the transponder. The information flows to the satellite in frames, each frame containing one burst of information from each earth station allowed access to the single transponder. A typical format for a frame is shown in Figure 7.22. The sum of the durations of the individual bursts does not quite equal the frame time in order to give some guard time between bursts.

Timing for keeping the station bursts apart is a major problem, and it is complicated by two facts. First, the satellites are not perfectly stationary in orbit (each appears to travel in a small figure-eight pattern). Second, the times of travel of the signal between different earth stations and the same satellite are different because of different slant range distances. TDMA techniques allow the satellite transmitter to be operated at higher power levels than FDMA. This method is permissible because only one carrier at a time occupies the transponder, reducing the amount of intermodulation distortion that's generated.

Figure 7.22: TDMA transmission frames.

Demand Assignment Multiple Access

Using the Demand Assignment Multiple Access (DAMA) technique, each satellite transponder is used much like a telephone switch; that is, a subchannel is assigned only when traffic is available to be carried on it. This is in contrast to the FDMA and TDMA techniques described previously, in which channels are assigned to users permanently, even if there is no traffic demand. DAMA is a variant of FDMA in that a part or all of the transponder is divided into individual channels that can be accessed by all DAMA terminals on the ground that are serviced by the DAMA transponder. A central computer system, or a system of distributed and cooperating computers, on the ground keeps track of who gets which channel when. DAMA is particularly useful for loading transponders efficiently when each of several ground sites needs only a few channels.

Personal Communication Services

During the 1970s and 1980s, satellite communications were often thought of as the data communications technology of the future. In fact, many authors wrote about the use of satellite technology eventually replacing the use of most terrestrial communications. This conversion did not, however, occur primarily due to the delays associated with transmitting a signal more than 25,000 miles into the atmosphere and then having that signal retransmitted back to earth. The delay time associated with the use of satellites significantly affects their use for data transfer when data blocks have to be periodically acknowledged. Due to the minimal effect on voice conversations, the use of satellites has taken on a new role as a mechanism for personal communication services.

The key difference between the evolving personal communications satellite-based systems and existing cellular telephones is in their method of communication. A cellular telephone user communicates with a cell site connected via terrestrial lines to the public switched telephone network. In comparison, although initial PCS offerings provide cellular-like voice services using different frequencies to transmit to cell stations, ground users can also subscribe to a low earth orbit satellite-based communications service. Such subscriptions enable ground users to make cellular calls or receive pages through low earth orbiting satellites from just about every area on earth.

Several companies are currently developing satellite systems consisting of 12 to 60 or more non-geostationary low and middle earth-orbiting satellites that will provide either continental or near-global communications coverage. Such systems are being developed to support a new type of wireless telephone service that will transmit in the L and Ka-band frequency spectrums and support high-quality voice service as well as facsimile and paging signal transmission.

One of the first satellite-based personal communications services is the Iridium system from Iridium, Inc. This firm represents a consortium of telecommunication operators and industrial companies originally sponsored by Motorola, Inc., which still has a minority interest in this venture. The actual concept behind Iridium dates to 1985 when Karen Bertiaer, the wife of a Motorola executive, could not place a cellular call to the United States when vacationing in the Caribbean. She convinced her husband of the need for a global mobile wireless system. Initially, Iridium planned to launch 77 low earth orbit satellites but later they scaled back their plan to 66. Although the name of the consortium was based upon the 77th element, it fortunately was not changed. Otherwise we might refer to the consortium as Dysprosium for the 66th element, which translates to difficulty of access! The first Iridium satellite flight bus was delivered by Lockheed Martin to Motorola Satcom facilities during October, 1995. By 1997, Iridium had placed 47 satellites into orbit and by 1998, the constellation of 66 satellites circling the earth was completed.

The Iridium satellite network consists of 66 satellites in six orbital planes approximately 480 miles above the earth. Communications to Iridium satellites are via special Iridium mobile telephones and via conventional telephone systems through the use of satellite earth station gateways that convert terrestrial signals to the Ka-band for transmission to an Iridium satellite. In addition to Iridium, other major communications firms provide competitive satellite-based personal communications services. One such competitive service is provided by ORBCOMM, a partnership owned by Orbital Sciences corporation, Teleglobe, Inc. of Canada, and Technology Resources Industries of Malaysia. ORBCOMM currently has 36 satellites in orbit and during 1998 received permission from the FCC to add another 12 satellites to its constellation. ORBCOMM provides a variety of two-way data and messaging communications. Applications include monitoring of fixed assets such as electric utility meters, pipelines, heavy equipment, rail cars, and other vehicles.

Low earth orbit satellites are ideally suited for two-way cellular and paging operations. This is because the low orbit of those satellites requires less power to reach those satellites. Anyone who has seen an old Tom Clancy made-for-TV movie in which an intelligence agent opens a suitcase and constructs a small earth station to bounce a signal off a satellite can now recognize that the person was accessing a geostationary satellite. In comparison, in the movie "Air Force One," Harrison Ford was able to use a small handheld cellular phone because he was accessing a low earth orbit satellite.

In the current frequency assignments, how many frequency bands are there for the uplink frequencies?

16

8

4

2

How much bandwidth is required to send 132 voice-grade channels by FDM on an international satellite system?

500MHz

10MHz

1320MHz

50MHz

What is Iridium?

A geostationary series of 77 satellites.

A low earth orbit series of 77 satellites.

A geostationary series of 66 satellites.

A low earth orbit series of 66 satellites.

Which of the following is a primary advantage of low earth orbit satellites for two-way communications?

Ground locations

Power to access

Weather

Immunity to solar flares

About the Author

Gilbert Held is an internationally known award-winning lecturer and author. He is the author of more than 40 technical books and 300 articles covering the fields of personal computing and computer communications.

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